As part of a routine maintenance program, it is often necessary to clean and/or decontaminate tools and pieces of equipment which have become "dirty" by virtue of their everyday use in a contaminated environment. For example, in a nuclear power plant or other reactor facility, tools, utensils and machine parts may come in contact with radioactive liquids, dust, aerosols, and the like which may adhere to their surfaces so that after a period of time, they may become sufficiently contaminated to present a radiation hazard. To avoid this problem, these items would be cleaned periodically so that the radiation which they emit is maintained below an acceptable level or count. The same problem arises with biologically and chemically contaminated parts.
Various techniques have been used to clean the surfaces of contaminated objects. These include blasting the object surfaces with water or grit and cleaning the surfaces with freon or other chemicals. These prior techniques are disadvantaged because they create a secondary waste problem because the dirt and contamination on the object being cleaned becomes entrained in the cleaning medium which then has to be disposed of as secondary waste.
There does exist a dry process which cleans and decontaminates by blasting the object to be cleaned with particles capable of sublimation, for example, carbon dioxide (CO.sub.2) particles. This process, described, for example, in U.S. Pat. Nos. 4,038,786 and 4,389,820, uses solid carbon dioxide particles or pellets propelled by dry compressed air. The CO.sub.2 particles shatter upon impact with the surface to be cleaned and flash into dry CO.sub.2 gas which penetrates the surface pores and flushes out any dirt or contamination therein. The CO.sub.2 particles do not abrade or attack the surface of the object being cleaned. Consequently, the process can be used to clean hard objects made of metal or the like, as well as softer objects made of rubber, wood, plastic, etc. Advantageously also, since the process relies on a material which sublimates or gasifies while cleaning, there is no accumulation of contaminated particulate matter or chemicals that would require disposal as hazardous waste.
It has been proposed to use the CO.sub.2 blasting process to satisfy the parts cleaning and decontamination procedures that have to be carried out routinely in this country's nuclear power plants and similar facilities. However, attempts to adapt or accommodate such a system to these standard procedures have not proven to be too successful because of the cost involved. More particularly, the standard decontamination routine at this country's nuclear facilities requires the establishment of closed rooms for the treatment of tools and equipment which present different degrees of danger to the decontamination team and to others. For example, in a simple case, the facility may include one room for cleaning or decontaminating relatively large parts which have a high overall radiation count. The personnel working in that room and cleaning those parts may have to be completely enclosed in protective clothing with self-contained breathing equipment. A second room of the decontamination facility may be devoted to cleaning small parts and tools that present a lesser radioactive hazard. The contaminated parts may be brought into the second room with the actual cleaning of the parts being carried out in a sealed decontamination cell or glovebox in the second room so that the personnel working in that room do not have to wear protective clothing other than, say, a lab coat.
Other areas of the facility may be dedicated to cleaning other categories of equipment. Invariably also, each facility includes a so-called clean or count room where the decontaminated parts may be checked or "frisked" with a radiation counter to verify that they are indeed clean before the parts are returned to service. Personnel in this area are normally exposed to minimal radiation and may, therefore, wear street clothes. Not only must certain safety procedures be carried out in each different room of the contamination facility, but also strict routines must be followed when moving from room to room to insure that contaminants in one room are not transported into a cleaner room. For the same reason, the facility's ventilation system must be designed to prevent contaminants from being entrained in the air circulating between rooms.
Also, if decontamination is to be conducted using the aforesaid CO.sub.2 pellet blasting system, there must be an additional relatively large room in the facility devoted to the pellet making machine and the equipment required to produce the dry air stream to propel those pellets against the objects to be cleaned.
The net result is that a permanent decontamination facility that cleans by CO.sub.2 blasting which is only used on a periodic basis, e.g. every six months, is very expensive to maintain. First, it occupies a relatively large amount of ground space which is usually at a premium at most reactor facilities. Also, the building itself is invariably quite costly because its various rooms, particularly the ones used for decontamination, require, in accordance with accepted practice, walls faced with stainless steel panels which can be cleaned easily and with special seams between the panels to prevent leakage from the rooms of airborne radioactive material. Finally, the apparatus for making and propelling the CO.sub.2 pellets is relatively expensive so that it is not cost effective to leave the apparatus on-site and use it only every six months or so.
Resultantly, at those reactor facilities where CO.sub.2 blasting decontamination is performed, it has been the practice to transport the pellet-making apparatus and ancillary equipment to the jobsite when decontamination is due, erect a completely new decontamination facility, carry out the decontamination program and then tear down the building and transport everything from the site. Invariably, those portions of the building, i.e. walls, ceiling panels, etc., exposed to radiation, are considered hazardous waste and have to be disposed of accordingly. Obviously, decontamination on this "hit and run" basis is also very costly both in terms of manpower and materials. Moreover, even though the decontamination process by CO.sub.2 blasting does not create radioactive waste directly, the destruction of the decontamination facility upon completion of the job does, as just stated, result in secondary waste.
It has also been found that conventional CO.sub.2 blasting systems are not particularly adapted to clean or decontaminate parts in an isolated environment, i.e. inside a decontamination cell or glovebox. This is because it is difficult if not impossible to aim the apparatus' discharge nozzle with sufficient accuracy to enable the CO.sub.2 pellets issuing from the nozzle to properly scrub all areas of the part being processed. Resultantly, the decontamination of small and intricate parts using standard CO.sub.2 cleaning equipment tends to be tedious and time-consuming.